1. Field of the Invention
The present invention relates to methods and apparatuses for detecting objects around vehicles, and more specifically, to a method and apparatus that are capable of accurately detecting an obstruction on a road.
2. Description of the Related Art
A radar device mounted in a vehicle, such as a millimeter-wave radar device, may be used to detect a preceding vehicle. This function may be utilized for speed control using an accelerator or a break to keep a space between vehicles in fields of an anti-lock brake system (ABS) and cruise-control techniques, and also used for steering control. Locations where such a radar device is utilized include straight roads such as freeways, on which vehicles can travel at high speed. Such radar devices are designed so that its receiving sensitivity can detect vehicles traveling ahead in the same direction and oncoming in the opposite direction and obstacles at sides of roads, such as guardrails, and signs.
Japanese Unexamined Patent Application Publication No. 10-221451 discloses a manner of determining whether an object ,such as a preceding vehicle or a roadside structure, detected by a vehicle-mounted radar device is an obstruction. Specifically, a relative location of an object with reference to a vehicle is calculated on the basis of signals supplied by a vehicle-mounted radar device, an absolute location of the object based on the relative location and a current position of the vehicle is found, the absolute location is compared to location data on a roadside structure, and thereby the object is thereby determined to be or not be a roadside structure.
The vehicle-mounted radar devices, such as millimeter-wave radar have a fixed receiving sensitivity. Therefore, when the receiving sensitivity is set at a rather high level (a threshold of strength of received radio waves for detecting an object is set at a rather low level), as illustrated in FIG. 11A, preceding vehicles 1 and 2, a sign 3, and guardrails (not shown) can be detected and displayed on a radar screen. On the other hand, when the receiving sensitivity of the vehicle-mounted radar device is set at a rather low level, as shown in FIG. 11B, the preceding vehicles 1 and 2 can be detected, but the sign 3 and the guardrails cannot be detected. Accordingly, the receiving sensitivity of the vehicle-mounted radar device is set at a level of FIG. 11A that is sufficient for detecting the sign 3 and the guardrails.
However, detecting conditions vary with surroundings; for example, as shown in FIG. 11C, when a vehicle travels on a section of a road with sound barriers, a walled-in area, such as tunnels, and a place enclosed with a steel material, such as bridges, increased radio waves reflected from the wall are picked up and output on a radar screen as noise (circles 4 on the radar screen). As a result, the detection of nearby vehicles becomes difficult.
The device of Japanese Unexamined Patent Application No. 10-221451 requires distinguishing whether a detected object is a scanning target (another vehicle) or an object not to be scanned (a roadside structure), and it is necessary to compare an absolute location with location data on the roadside structure. This comparison operation places a heavy load on the radar device, resulting in a problem in which obstructions traveling at high speed cannot be detected.